1. Field of the Invention
The present invention relates to electrophotographic copying machines, and, in particular, to electrophotographic wherein dust and the like foreign matter are prevented from adhering to the component elements.
2. Description of the Prior Art
Among the various types of electrophotographic copying machines, there is a system where an electrostatic latent image is formed on a photosensitive member comprising a conductive layer and a photoconductive layer, and then the electrostatic latent image is transferred onto a recording member such as electrostatic recording paper, followed by the step of developing the recording member, as in the case when the T.E.S.I. method is employed. In copying machines of the electrostatic latent image transferring type as above mentioned, the surface of the photosensitive member is not subjected to development, which is different from copying machines utilizing the xerographic method, wherein charging, exposure and developing are carried out on the photosensitive member and then the developed image is transferred onto a recording member. On the other hand, as in the case employing the ion flow modulating method, there is another system in which use is made of an ion flow modulating grid (hereinafter refered to as "photosensitive screen") comprising a conductive layer and a photoconductive layer and provided with a number of fine pores, whereby an electrostatic latent image is formed on the photosensitive screen and then another latent image in accordance with the image on the screen is formed on a recording member, positioned apart from the screen. The recording member bearing thereon as thus formed an electrostatic latent image based on the above-mentioned ion flow modulating method, is subsequently subjected to development. In either of these systems, since there is no need to use cleaning materials such as for removing excessive toner on the surface of a photosensitive member or photosesitive screen having an electrostatic latent image formed thereon, especially in the case of the system employing the ion flow modulating method where it is unnecessary to contact the recording member is, there exists no chance that the photosensitive surface undergoes machanical damage and, therefore, it is expected that the photosensitive surface can be presented for use for a long period of time under a stable condition.
However, if such a photosensitive member or photosensitive screen having an electrostatic latent image formed thereon as above mentioned, has been subjected to a corona discharge for a long period of time, differences in the surface condition occur. It often occurs that it becomes virtually impossible to form an electrostatic latent image on such a surface. The reason for this is that dust-state, fine particles of 0.1 to 50 u, predominantly consisting of oxidized silicone drifting in the air, come to bear charges due to corona ions and adhere to the surface of the photosensitive member and the screen, causing a decrease in the surface resistivity of the photosensitive member. This phenomenon has been determined empirically. If use is made of a photosensitive member, the surface resistivity of which has decreased as above described, it is no longer possible to charge the surface of the photosensitive member to a sufficient degree and the contrast of an image reproduced deteriorates. Moreover, if this phenomenon of decreasing the surface resistivity of the photosensitive member continues, it comes to be impossible to obtain a reproduced image, which is endurable in practical use. Particularly, in case of the above-mentioned photosensitive screen having numerous pores, it is difficult to apply cleaning means for removing dust from the surface thereof. In the xerographic system, no such problem as the accumulation of dust on the photosensitive member occurs because developing and cleaning are carried out on the surface of the photosensitive member.